Illaoi bug with Tryndamere Ult

I was playing Tryndamere today, and both times I 1v1ed Illaoi and used my ult, her tentacles ignored my ult and killed me. I didn't believe it the first time, so I found her and tried it again, and sure enough, my ult didn't do anything against Only her tentacles. Her autoattacks didn't bring me under 1 health, but her tentacles finished me off with ease.